UIFe for Subiquity - Mirror selection page

[Olivier Gayot]

Hello folks,

In order to fix an Ubuntu Server installer bug, the release team approved a UIFe in Subiquity.
The change is only targeted for the text-based installer (Ubuntu Server). Other installers that use
Subiquity in the backend (e.g. Ubuntu Desktop Bootstrap and the installer for some of the flavors)
are not affected by the change.

The UI change (which I expect will land shortly) is minimal but will require existing 
translations to be adjusted accordingly.

Code to be merged (includes screen captures of the updated UI):
-----------
https://github.com/canonical/subiquity/pull/1968

LP Bug link
-----------
https://bugs.launchpad.net/ubuntu/+source/subiquity/+bug/2059898

Thank you,
Olivier